1. Field of the Invention
The present invention relates to a novel electrifying method and to an electrifying apparatus used therefor. More specifically, the invention relates to a method of uniformly electrifying a material to be electrified such as a photosensitive material or like materials without causing the surface thereof to be damaged or worn out and to an apparatus used for the above method.
2. Description of the Prior Art
An electrophotographic apparatus is employing a system which forms an electrostatic latent image by uniformly electrifying the surface of a photosensitive material, and exposing the thus electrified photosensitive material to the light bearing image.
A corona wire has heretofore been widely used for electrifying a material that is to be electrified such as a photosensitive material and the like materials. The corona electrification, however, requires a high-voltage generating device for generating a corona discharge which causes the electrifying apparatus to become expensive. Moreover, the corona discharge involves the generation of ozone and harmful components such as NO.sub.x and the like, which are hazardous to the environment and deteriorate the photosensitive material.
In order to solve the above problems, there have been proposed a variety of so-called contact electrification systems which perform the electrification by bringing an electrification mechanism and a photosensitive material into direct contact with each other. These systems can be roughly divided into a brush electrification system, a roller electrification system, and a blade electrification system.
A brush electrification system has been disclosed in Japanese Patent Publication No. 220588/1985 according to which the electrification is effected by bringing an electrically conducting brush having electrically conducting contactors into contact with a photosensitive material, the contactors having a resistivity of 100.sup.0 to 10.sup.4 ohms.cm, a thickness of 3 denier (d) to 6 d, and being provided at a density of 50,000 to 200,000 hairs/square inch.
As for a roller electrification system, Japanese Unexamined Patent Laid-Open No. 149668/1988 discloses a contact electrification method which performs the electrification by applying a voltage to an electrically conducting member that is in contact with the surface of a material to be electrified, wherein said electrically conducting member has a surface region that comes in contact with the surface of the material to be electrified and a surface region which is continuous thereto and gradually separates away from the surface of the material to be electrified, as it goes toward the downstream side in the direction in which the surface of the material to be electrified moves, and wherein a pulsating voltage having an inter-peak voltage which is more than twice as great as the electrification start voltage is applied to said electrically conducting member, thereby to establish an oscillation electric field between said separating region of said member and the surface of the material to be electrified and, thus, to electrify the material. Moreover, Japanese Unexamined Patent Laid-Open No. 19116/1989 discloses a roller that is formed between the electrically conducting lower layer and the upper layer having a high resistance.
As for a blade electrification system, Japanese Unexamined Patent Laid-Open No. 203754/1991 discloses an electrifying device having an electrifying blade with its tip being opposed to the direction in which the material to be electrified moves and is in contact with the material to be electrified, the electrifying blade being so disposed as to form a wedge-like gap relative to the material to be electrified, and the electrifying blade further having a resistor that serves as an electric discharge electrode which will be impressed with a voltage at a portion opposed to the material that is to be electrified.
The above-mentioned contact electrification systems have a merit in that they require application voltages lower than that of the corona electrification system, and generate neither ozone nor NO.sub.x, but have a defect in that the electrification is not so uniform as that accomplished by the corona electrification system.
That is, in the case of the brush electrification, the contact between the electrifying member (brush) and the material to be electrified is a point contact or a line contact; i.e., when viewed microscopically, there develop portions which are not electrified causing the electrification to become nonuniform. Moreover the brush that comes into direct contact with the surface of the photosensitive material causes the photosensitive material to be worn out. The tendency of wear appears conspicuously when the brush is rotated at a high speed in an attempt to accomplish uniform electrification. Furthermore, the brush has a short life since it is subject to be worn out, or loses hair and performance.
In the case of the electrification system using an electrically conducting roller, any ruggedness in the photosensitive material makes it difficult to bring the recessed portions into uniform contact with the roller despite a line pressure is given to the roller, making it still difficult to accomplish uniform electrification. Generally, dust, paper powder and toner are adhered to,the surface of the photosensitive material. Presence of foreign matter between the roller and the photosensitive material. The presence of foreign matter between the roller and the photosensitive material causes the electrification to become defective in the direction of width. If a large pressure is given to the roller in an attempt to solve the above problems, however, the result is that the photosensitive material is subject to be worn out conspicuously. It may be contrived to impart soft cushioning property to the surface of the roller. However, the electrically conducting rubber has a limit in its softness and, besides, a softening agent (plasticizer) in the rubber migrates onto the surface to contaminate the photosensitive layer.
Even in the-case of the blade electrification system like in the case of the roller electrification system, the electrification is not only not uniformly accomplished but this tendency rather becomes more conspicuous due to blade inversion (turn up) and cracking from the friction by the surface of the photosensitive material. Accordingly, the photosensitive material is more worn out and damaged.